Cleavage of Carbon Halogen Bonds

A bond between carbon and halogen can be cleaved quite easily with light221 One of the few synthetic applications of this reaction is the photo-arylation of aromatic compounds, a sequence employed in the synthesis of alkaloids222,223) as shown for the key step in the synthesis of boldine (2.13) 223 . 

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Step (1) Alkyl halide dissociates by heterolytic cleavage of carbon-halogen bond (Ionization step)... 

Electrogenerated nickel(I)251 and cobalt(I)252 complexes of Salen (Salen = bis(salicylidene)ethane-1,2-diamine) have displayed good catalytic properties in the cleavage of carbon-halogen bonds in a variety of organic compounds. Recent research in this field has been reviewed.253... 

The catalysis of the cleavage of carbon-halogen bonds by complexation with metal ions such as silver or mercuric ion is a well-known phenomenon. The compounds susceptible to this action are alkyl halides capable of forming car-bonium ions. The complexed anions such as in mercuric nitrate, mercuric perchlorate, or hydrated mercuric ion do not exhibit a simple relationship between their effect on the total rate and on the relative distribution of products stemming from water or the anion. This evidence is indicative of the following catalytic mechanism ... 

The reductions of halogenated organic compounds (RX) involve the cleavage of carbon-halogen bonds [62]. Depending on the solvent, supporting electrolyte, electrode material and potential, it is possible to electrogenerate either alkyl radicals (R ) or carbanions (R ), which then can lead to the fonnation of dimers (R-R), alkanes (RH) and olefins [R(-H)] ... 

Reductive cyclization of a/p/M-bromoacetamides and 3-bromopropanamide to beta-lactams has been performed by electroreduction in the presence or absence of a probase 41). The cyclization seems to involve the reductive cleavage of carbon-halogen bonds generating carbanions which would act as nucleophiles or bases in the electrolysis media. 

Peters and Sweeny carried out cyclic voltammetry of nickel(ll) salen at a glassy carbon electrode in [BMlMJiBFJ. They found that nickel(ll) salen exhibits one-electron, quasi-reversible reduction to nickel(l) salen, and that the latter species serves as a catalyst for cleavage of carbon-halogen bonds in iodoethane and... 

Reduction of aromatic and polyethylenic ketones —pinacols Cleavage of carbon-halogen bonds ... 

Analogous mechanisms are applicable to the splitting of other saturated molecules and are realized in the following examples involving the reductive cleavage of carbon-halogen bonds (39, 40, 45, 51, 54). 

The cleavage of carbon-group-VIB, -VB or -IVB element bonds to give organolithiums is directly analogous to the Li cleavage of carbon-halogen bonds (see 5.5.2.2.1) ... 

The characterization of reaction mechanisms and intermediates of complex chemical reactions forms a central topic in electrochemistry [113-118]. The electrochemical reductive cleavage of carbon-halogen bonds is an important process in electro-organic synthesis, waste stream treatment and electron-transfer mechanisms. Benzyl chloride (PhCH2Cl) reduction in organic solvents has been widely investigated following debate as to whether the carbon-chlorine bond would be reductively cleaved via a concerted or a nonconcerted reaction pathway. Recent studies have shown that Pd, Cu - and especially Ag - cathodes... 

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